Comcast@Home says call 1-800-782-0398 and mention Offer code Q400B (works for all), and they will send you a $100. Gift Check in 6-8 weeks after sending in a copy of your bill to the specified address. They state the Gift Check is good for online shopping, but couldn't specify if it was limited to certain retailers, but hey, $ is $, and I checked their site and there are no details as to which retailers it is with either. You sign up, get a month free, free installation or free self-install kit (with the modem), free one month of service with no commitment. Offer applies to standard installation for new residential customers in Comcast Cable wired and serviceable areas only.
Here is the Gift Check Info.
Here is the Gift Check Info.